Wireless communication systems generally consist of subscriber units which communicate via radio frequency (RF) communication with one or more base stations. In terrestrial cellular systems, the base stations are fixed and the subscriber units may be mobile. In some satellite communication systems, the satellites operate as base stations and are moving while the subscriber units may be fixed or mobile.
To initiate communication between the satellite communication system and the subscriber unit, the subscriber unit must acquire timing information from a satellite and synchronize its timing with satellite or system timing. To accommodate this, each satellite in the system periodically transmits bursts of a predetermined data pattern. Upon detection of a burst, the subscriber unit identifies the satellite system and synchronizes to system timing. Upon synchronization, the subscriber unit may begin communication with the system. A similar process occurs in terrestrial cellular systems.
The system acquisition process is made complicated by the nature of satellite systems. A satellite transmits a number of spot beams to the surface of the earth. Not all information, such as synchronization information or subscriber-specific alert information, is transmitted on all spot beams. Therefore, the subscriber unit must know or be able to predict which spot beams to scan to obtain synchronization information. Also, because of the substantial distance between satellites and terrestrial subscriber units, the subscriber unit must accommodate propagation delays as signals travel between satellite and subscriber unit.
Lastly, many subscriber communication devices for use with satellite communication systems, such as radiotelephones and pagers, are portable, battery powered devices. Since battery power is limited, conservation of such battery power is highly desirable. In particular, it is desirable to minimize battery power usage when the communication device is in a standby mode, acquiring and tracking system timing.
Accordingly, there is a need in the art for a method for accquiring system timing and synchronization which minimizes battery power consumption.